"I had finished the marathon and was walking back to see my family," Victoria’s Johnny Greaves told CBC News. "I heard two big explosions and I saw smoke," he said.

Of the nearly 27,000 people participated in the marathon this year, about 2,000 of them were from Canada and 232 were from British Columbia.

Foreign Affairs officials told CBC News that so far, there have been no reports of any Canadians injured in the blasts.

Vancouver’s Rob Watson crossed the finish line early on, finishing 11th overall. He was having lunch in a nearby hotel when he heard the first explosion go off across the street, about 200 metres away.

"It sounded like a very loud thunder strike, you know, and we looked out the window and it was sunny out, so it seemed very odd," Watson said. "And then about 10 seconds later, there was another one."

White Rock’s Jim Millington wasn’t able to finish the marathon. Race officials held him up as he was minutes away from the finish line.

"You know, there were just cops all over the place and sirens, and I guess I don't know if it's a terrorist attack or what it is," he said.

'Potential terrorist investigation'

The blasts occurred about three hours after the winners crossed the line. The first explosion was on the north side of Boylston Street, just before the photo bridge that marks the finish line. The second went off seconds later just down the street.

Competitors and race organizers were crying as they fled the chaos. Bloody spectators, including some who have severed limbs, were being carried to the medical tent that had been set up to care for fatigued runners.

Another B.C. runner was collecting her medal when she heard the blasts. Susan Danard said she immediately thought there had been an accident.

"Yet the American runners who were with me immediately thought it was a criminal act or terrorism," Danard told CBC News.

"In fact, one of them started crying, and someone told me she was from New York and this had probably triggered back some very bad memories."

On Monday afternoon, U.S. President Barack Obama spoke to reporters in a press conference.

"We still do not know who did this and why, and people shouldn’t jump to conclusions before we have all the facts," he said. "But make no mistake, we will get to the bottom of this. And we will find out who did this, we will find out why they did this."

Boston officials said the incident was a criminal investigation and a potential terrorist investigation.

"The Boston Marathon is such a celebration and people are just getting together and having just a great day," said Watson, one of the runners from B.C.

"It's such a positive event for the city of Boston. And for some people to do something like this, it's just saddening and it's sickening and it's just very frustrating."

Sun Run officials to meet Tuesday

Sun Run officials said they would meet with city officials, firefighters and police on Tuesday to review their security plan for the event this weekend.

Spokesman Jamie Pitblado encouraged runners to participate on Sunday's Sun Run and said a solid emergency preparedness plan is in place for the run.

"The worst thing that we could do is not show up and let these people who are causing these issues kind of take over our streets and put fear in our minds and in our hearts," Pitblado told CBC News.

Canadians in Boston

Canadians in need of help in Boston can contact the Canadian Consulate at 617-247-5100 or email sos@international.gc.ca.

Canada's Department of Foreign Affairs has also has an emergency line — 1-800-387-3124 — to help Canadians in Boston.

"I think it's very important that we get out and show our support for our community and for each other and ultimately, I guess, we could show support for those who have been affected by Boston."

With the BMO Vancouver Marathon scheduled for May 5, officials from that event released a statement on Monday.

"The Vancouver International Marathon Society has worked extensively with City stakeholders, including police, fire, medical and others, to develop a solid emergency plan," the statement read.

"We want to reassure runners and our extended running community that safety and security are our top priorities."

B.C. Liberal Leader Christy Clark commended the first responders in Boston for their "quick and professional actions."

"Regardless of the cause, it's a dreadful thing to happen — particularly near the very spot where runners and spectators from around the globe have been celebrating athletic achievement for the past 117 years," Clark wrote in a statement.

"Our government stands ready to offer whatever assistance may be needed in light of this tragedy."

Boston Marathon explosion updates

CBC News readers can follow live updates on the events in Boston below: